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The Nano-Spintronics-Cluster-Tool

The Nano-Spintronics-Cluster-Tool is a dedicated experimental platform for magnetism- and spintronics-related activities in our institute and integrates various complementary methods into a single instrument within an ultrahigh vacuum environment (UHV).

The Nano-Spintronics-Cluster-Tool is a dedicated experimental platform for magnetism- and spintronics-related activities in our institute and integrates various complementary methods into a single instrument within an ultrahigh vacuum environment (UHV).

A central part of the Nano-Spintronics-Cluster-Tool is a high-resolution scanning electron microscope (SEM). The spatial resolution limit at high beam energies (30 keV) is 3 nm, while a beam booster improves the resolution at low beam energies (down to 0.1 keV). A high beam current of up to 5 nA in combination with an electron spin polarization detector allows fast structural and magnetic studies (SEMPA) to be carried out on the mesoscale at variable temperatures (700 down to 30 K).

Finally, the system provides three approaches for in-situ structuring: (i) shadow deposition through microapertures for contact pads, (ii) focused ion beam (FIB) etching in dual-beam configuration with the SEM for writing arbitrarily-shaped structures of sizes down to 20 nm, and (iii) scanning tip structuring by STM for length scales below 20 nm. Thus, 6 orders of magnitude in sample size can be covered. The STM and SEM stages feature four electric contacts to the sample, which allow in-situ electric transport measurements.

Figure: Top view of the Nano-Spintronics-Cluster-Tool: The left chamber houses the SEM, the spin polarization detector, and the FIB. The chamber on the right side contains the low temperature STM in the front part with the preparation and analysis chamber at the back. A transfer chamber (middle) connects the two units and enables in-situ access to all attached preparation and characterization tools without a vacuum break.

Results

1. Magnetic contrast with SP-STM

We tested the performance of the STM and the ability to achieve spin contrast by investigating thin (1–2 ML) Fe films on W(110), the standard test system for SP-STM [O. Pietzsch et al., Phys. Rev. Lett. 84, 5212 (2000)]. An in-situ grown Fe wedge is studied in an area where the Fe film is 1.5 ML thick and thus exhibits areas with one or two ML Fe coverage, respectively. We used an etched, chemically-wet tip made of antiferromagnetic bulk Cr. The figure shows the simultaneously acquired topography (a) and differential conductivity (dI/dV) map at E-EF=-300 meV (b) as well as a 3D representation of the topography coloured according to the spectroscopic data (c). The experiment was performed at 4.8 K.

The topography image (a) was measured in the constant-current mode with a gap voltage of -300 mV addressing occupied states and a tunnelling current of 2.5 nA. The scan area (180 nm x 180 nm) comprises 3 surface steps of the W(110) substrate and spans a total height difference of about 0.8 nm, including the islands on the topmost terrace.

The post-annealing treatment at 560 K causes the Fe atoms in the second ML to diffuse to the step edges, where 2 ML high Fe stripes are formed [red marks in (c)]. Dislocation lines in the 2 ML areas [red arrow in (a)] run along the [001] direction. The simultaneously recorded dI/dV map at E-EF=-300 meV in (b) displays strong contrast in the 2 ML areas only. The dislocation lines appear as bright lines running along [001].

The alternating brown/yellow contrast is due to magnetic domains of the out-of-plane magnetized 2 ML-thick Fe film. The first Fe ML on W(110) is in-plane magnetized and does not contribute to the magneticcontrast of this image. The conductivity map was measured by lock-in detection at a frequency of 2700 Hz and a peak-to-peak modulation amplitude of 42 mV. The overlay of topography and the dI/dV map in (c) gives a visual impression of the subtle nanoscale structure-magnetism correlation.

We have developed a novel multi-stage fabrication process for lateral spin valves that is performed completely in-situ in order to realize clean and well-defined interfaces. In addition, the ferromagnetic layers are topmost and thus accessible to surface sensitive techniques (e.g. SEMPA). The process is based on thermal evaporation and structuring with UHV-FIB. Our spin valves comprise two ferromagnetic Co wires of different length that are attached to Cu leads.

For the non-local transport measurements, a current I is applied between C1 and C4 in order to create spin accumulation in the Cu lead between C1 and C2. The pure spin current signal is detected as a voltage VS across C2 and C3, which depends on the relative magnetic alignment of FM1 and FM2..

Figure: Pure spin current signal RS=VS/I measured for two different excitation currents. The two levels in each measurement correspond to the parallel and antiparallel alignment of the magnetizations in FM1 and FM2. The magnitude of the effect of about 1 mΩ indicates clean interfaces.

The non-reproducible switching (e.g. at negative magnetic field) is due to the formation of magnetic domains in the Co wires, which are stabilized by ion-beam induced roughness of the substrate. The SEMPA measurements indicate the formation of the so-called concertina domain structure.

Figure: SEM topography and corresponding SEMPA image of the ferromagnetic wires. In spite of their dimension (10 nm thick, 220 to 280 nm wide, and approximately 4 µm long) they reveal a rich domain structure similar to the schematically-shown concertina structure.

Original Publication:

3. Accessing 4f-states in single-molecule spintronics

Magnetic molecules are potential functional units for molecular and supramolecular spintronic devices. However, their magnetic and electronic properties depend critically on their interaction with metallic electrodes. Charge transfer and hybridization modify the electronic structure and thereby influence or even quench the molecular magnetic moment. Yet, detection and manipulation of the molecular spin state by means of charge transport, that is, spintronic functionality, mandates a certain level of hybridization of the magnetic orbitals with electrode states.

We have shown how a judicious choice of the molecular spin centres determines these critical molecule–electrode contact characteristics. In contrast to late lanthanide analogues, the 4f-orbitals of single bis(phthalocyaninato)-neodymium(III) molecules (NdPc2)adsorbed on Cu(100) can be directly accessed by scanning tunnelling microscopy. Hence, they contribute to charge transport, whereas their magnetic moment is sustained as evident from comparing spectroscopic data with ab initio calculations.

4. Deposition of NdPc2 on differently reactive surfaces

Realizing spintronic devices based on magnetic molecules requires that the molecules are adsorbed on substrates and/or electrodes. Thereby, the substrate-molecule interaction possibly modifies the structural, electronic, and magnetic properties. We have addressed the question about the stability NdPc2 molecules on differently reactive substrates such as Au(111), Cu(100), and 2 atomic layers Fe on W(110). We found different fractions of intact double-decker molecules on Fe/W(110) and Cu(100) and only decomposed single-decker Pc fragments on Au(111). Hence, the NdPc2 stability increases with increasing molecule-substrate interaction.

We explain this finding by charge transfer from the substrate to the lower Pc ligand of the NdPc2 molecule, which increases the intramolecular electrostatic interaction between the negatively charged Pc ligands and the Nd3+ ion. Stronger substrate-molecule interaction leads to larger charge transfer and thus increased stability of the double-decker molecule.

Figure: (a) Overview image after depositing NdPc2 molecules from a sublimation oven onto a Cu(100) surface showing some intact double-decker NdPc2 molecules (b) and a larger number of single-decker Pc fragments (c). The fraction of intact double-decker NdPc2 molecules depends on the substrate material and increases with the corresponding adsorption energy (d).

Exchange systems comprising a FeRh layer showing a temperature-induced metamagnetic transition from antiferromagnetic (AFM) to ferromagnetic (FM) and a hard magnetic layer are intensively discussed as a promising approach for heat-assisted magnetic recording that can largely increase the storage density of hard disk drives.

Recent work has indicated the coexistence of both AFM and FM states in capped single-crystalline FeRh thin films on MgO. Domain structures observed at the interface between the FeRh film and the capping layer revealed FM order, whereas the bulk of the FeRh film was in the AFM state. However, from these experiments it is not clear whether the coexistence of AFM and FM order is an intrinsic property of FeRh films or an artifact due to capping or contamination layers.

We have prepared 10 nm thick single-crystalline FeRh films on MgO(100) by separate layer deposition of Fe and Rh and performed structural, chemical, and magnetic characterizations in-situ without capping layers. Thus, we studied the intrinsic properties of the single-crystalline FeRh films without exposure to air or additional cleaning steps.

The FeRh films exhibit the metamagnetic phase transition from AFM to FM below room temperature as indicated by in-situ MOKE measurements. Temperature-dependent domain structures imaged by SEMPA at temperatures between 122 and 450 K reveal that FM domains exist at the surface, while the bulk is AFM. The domain size changes drastically with temperature, which is related to a spin reorientation transition from out-of-plane to in-plane between 350 and 400 K. The results show that the previously observed coexistence of the FM state at the surface and the AFM phase in the bulk is an intrinsic property of (100) surfaces of single-crystalline FeRh thin films on MgO(100).

Figure: SEMPA images of an in-situ prepared FeRh thin film measured at different temperatures as indicated. Images are taken at the same sample spot (see circles) and in the sequence (a) to (h). Panel (i) shows the colour-wheel for the representation of the SEMPA data. Different colours indicate the magnetization direction and the colour saturation the magnitude of the spin polarization.